Batteries are largely divided into a primary battery and a secondary battery. The primary battery generates electricity by using an irreversible reaction, and thus is not reusable after being used once. The primary battery includes a dry cell, a mercury cell, a voltaic cell, and the like. Unlike this, the secondary battery uses a reversible reaction, and thus is reusable after being used by charging. The secondary battery includes a lead-acid battery, a lithium ion battery, a Ni—Cd battery, and the like.
The lithium ion battery, which is one of the secondary batteries, includes an anode generally made of carbon, a cathode generally made of a lithium compound, an electrolyte positioned between the two electrodes, and a wire connecting the anode and the cathode. Lithium ions in the electrolyte move toward the anode at the time of charging and toward the cathode at the time of discharging, and introduce a chemical reaction while extra electrons are desorbed from or adsorbed to the respective electrodes. In this procedure, electrons flow through the wire whereby electric energy is generated. The lithium ion battery has been described herein, but also in the cases of other secondary batteries, basic principles and structures thereof are the same only except that materials used for electrodes or electrolyte are different. That is, the secondary battery generally includes the anode, cathode, electrolyte, and wire as described above.
Here, the secondary battery may be composed of a single anode, a single cathode, a single electrolyte, and a single wire. However, generally, a plurality of unit cells each composed of a single anode, a single cathode, a single electrolyte, and a single wire are connected to constitute the secondary battery. That is, the plurality of unit cells as described above are included inside a secondary battery pack. Certainly, the respective unit cells are electrically connected to each other.
In general, the secondary battery includes a plurality of unit cells therein, and is constituted in a type where a pair of external terminal taps (i.e., a pair of taps functioning as electrodes per one battery, while anodes of the respective unit cells are connected to each other to thereby become one anode of the battery and cathodes of the respective unit cells are connected to each other to thereby become one cathode of the battery) connected to electrodes of the respective cells are exposed to the outside. In particular, as shown in Korean Patent Laid-Open Publication No. 2007-0053614, a stacked type lithium ion battery has a structure where a plurality of cathodes and anodes immersed in an electrolyte are stacked with a separator therebetween and electrically connected in parallel and in series. This secondary battery does not use single cathode and anode, but constitutes a battery as a single pack where a plurality of cathodes and anodes are connected.
However, in the case where a plurality of cathodes and anodes are provided, one surface of the cathode, which is an electrode positioned at the outermost side, is contacted with an anode, while the other surface of the cathode does not have a counter electrode. In this case, metal ions constituting a cathode active material come out from a coating of the cathode, at which a counter electrode is not present, and move to an anode positioned nearest. Therefore, an excessive amount of metal ions enter the anode, and as a result, a metal plating phenomenon occurs. This metal plating phenomenon may shorten the lifespan of the battery, deteriorate characteristics of the battery, and damage stability of the battery.